Newswise — Pioneer 10 and Pioneer 11 were true trailblazers. The twin spacecraft gave humanity its first close-up glimpses of worlds beyond the solar system's asteroid belt. But once they'd passed beyond the orbits of Jupiter and Saturn, it became clear that something was amiss. Both spacecraft seemed to be slowing down more than expected. This "anomaly" persisted for decades without a satisfactory explanation. The featured article explains how the mystery of the anomaly unfolded and what it took to finally solve it.